Bloodstained: Ritual of the Night

If you want to boost this achievement, here is a fast way :

First equip a one-handed sword and go just outside of Johannes place.

Go just below the wooden plateform and hit Down, Down-Right, Right + , you should hit both the "morte" and the bat.

It takes roughly 5 min from 0 to 100% to master the technique.

Here is a short clip to show you my method.

One of the earliest you can Master is the Flashing Air Kick which you can use with the Kung Fu Shoes. You can find a Journal telling you how to do it after the second save room on the Galleon Minerva but here’s the action necessary: Down, Down-Right, Right + (Down, Down-Left, Left + if facing left).

You can perform this three times consecutively for a powerful finishing strike on the third. Downside is that it costs MP so make sure you have enough for how many kicks you want to perform.

Perform this on any enemy you see, a bonus is that during the animation of the kick Miriam is invincible so you could stand in front of an attacking enemy and proceed with the combo without being interrupted. Additionally it can be used in the air, which will also enable you to extend an overall jump (you can use this to get the “Plunderer’s Ring” early in the second deck area of the map by chaining the kicks).

To check progress on the Mastery of a Technique, open the Pause menu and go Archives > Techniques. When it’s yellow you’re all good for that technique and you’ll also get this achievement.

Using the other guides didn't work for me. Not sure why. I must have missed something. But using swords and doing down, down right, right X did literally nothing but swing my sword. I got the achievement with Zangetsuto (Zangetsu's sword) which is obtained after fighting him a second time. I found the technique to block physical damage actually works. right right X. I did this at the very beginning of the game where the squids constantly respawn. (deck of the ship) They will do an aqua attack, and then jump over you. Do the technique when they're above your head. This took me about 3 minutes from start to finish. There is also a save spot close by in case you run out of stamina. I'm not taking anything away from the other guide authors, it just did not personally work for me. It happens, just an alternative method in case someone else finds themselves in a similar situation.

I think the easiest one is Thousand Blossoms. You can use it with Rapier, Stinger or Kazikli, just spam quickly on enemies to unlock the achievement